1. Field of the Invention
The present invention relates to a spark plug for an internal combustion engine.
2. Description of the Related Art
A spark plug for an internal combustion engine is installed in an internal combustion engine of an automobile or the like and is used for igniting an air-fuel mixture. The spark plug generally comprises a center electrode, an insulator provided on the outer side of the center electrode, and a cylindrical metal shell provided on the outer side of the insulator. In addition, the insulator has a rear end-side trunk portion formed on its rear end side, a large-diameter portion formed on a leading end side of the rear end-side trunk portion, and an intermediate trunk portion and a long leg portion formed on a leading end side of the large-diameter portion. The insulator and the metal shell are combined, for example, in a state in which the insulator is inserted in the metal shell, a rear end portion of the metal shell is crimped, and the crimped portion is retained at a rear end of the large-diameter portion (e.g., refer to JP-A-2003-257583 (corresponding to US 2003/0168955A1)).
The spark plug is exposed to a high temperature in a combustion chamber, and a high combustion pressure is instantaneously applied thereto during combustion of the air-fuel mixture. Accordingly, the metal shell and the insulator making up the spark plug must have sufficient gas-tightness so as to be capable of preventing the air-gas mixture from leaking outside the combustion chamber under high temperature and pressure conditions, as well as stability in a tightened state so as to be capable of withstanding the combustion pressure. If an engagement allowance of the crimped portion with respect to a rear end of the large-diameter portion is insufficient, a gap is likely to occur between the insulator and the metal shell. Consequently, the force with which the insulator is held by the crimped portion declines, possibly leading to a decline in gas-tightness. In addition, as the engagement allowance decreases, the holding force of the crimped portion declines, with the result that stability of the tightened state of the insulator with respect to the metal shell can possibly decline. Accordingly, it is important to secure a sufficient engagement allowance of the crimped portion.
Incidentally, in recent years, there has been a demand for compact-sized and small-diameter spark plugs. To realize a small-diameter spark plug, the metal shell may be made thin-walled or the diameter of the overall insulator may be reduced. However, adoption of either method leads to a decline in the strength of the spark plug, and there is a possibility that the spark plug cannot withstand the aforementioned harsh environment of the interior of the combustion chamber. Accordingly, a small-diameter spark plug may be realized while ensuring necessary strength by reducing the diameter of mainly the most thick-walled large-diameter portion of the insulator.
3. Problems to be Solved by the Invention
However, if the large-diameter portion is thus reduced in diameter, the difference in diameter between the large-diameter portion and the rear end-side trunk portion disadvantageously becomes relatively small. Consequently, the area of the rear end stepped portion of the large-diameter portion becomes small, and hence difficult to sufficiently secure an engagement allowance of the crimped portion. Accordingly, in this case, there is concern that it leads to a decline in gas-tightness and a decline in the stability of the tightened state of the insulator with respect to the metal shell.